Dynamic reorganization of actin cytoskeleton is accomplished by a coordinated regulation of assembly and disassembly of actin filaments. In multicellular organisms, various types of cells utilize different mechanisms to regulate actin filament dynamics. Especially in muscle cells, the actin cytoskeleton is differentiated into myofibrils, which are highly organized contractile apparatuses. The long-term objective of this project is to understand the mechanisms by which the actin cytoskeleton is reorganized into myofibrils during muscle development. The expected outcome of this research will provide insight into molecular basis of morphogenesis of cellular organelles. This proposed project will use the nematode Caenorhabditis elegans as a model system and explore the roles of actin depolymerizing factor (ADF)/cofilin in actin filament dynamics during muscle development. ADF/cofilin enhances actin filament dynamics by depolymerizing and severing actin filaments. In C. elegans, mutations in a gene encoding a muscle-specific ADF/cofilin isoform cause disorganized actin filaments in muscle cells. These studies have suggested that the muscle-specific ADF/cofilin is a critical factor to generate a dynamic state of actin filaments during myofibril assembly. The aims of this project are: (a) to determine the effects of the muscle-specific ADF/cofilin on actin filament dynamics in vitro, (b) to determine specific roles of severing and depolymerization by ADF/cofilin in myofibril assembly in vivo, (c) to determine the roles of modulators of ADF/cofilin.